TDD methods enable electronic devices to communicate with network devices in both receive (RX) and transmit (TX) directions. This is achieved by dividing a radio frequency communications channel into a number of time slots and rapidly alternating activity between transmission and reception. Hence, full duplex, i.e. simultaneous two way communication, can be emulated with a half duplex or simplex transceiver, i.e. a device that is limited to one way communication at any one time. Examples of TDD systems comprise: Time Division—Code Division Multiple Access (TD-CDMA) implementations of the Universal Mobile Telecommunications System (UMTS), including both synchronous and non-synchronous specifications; Time Division—Long Term Evolution (TD-LTE) communications systems; Advanced eXtended Global Platform (AXGP) networks; and Institute of Electrical and Electronic Engineers (IEEE) standard 802.16 (so-called ‘WiMAX®’).
In TDD methods the use of a common frequency for both transmission and reception operations can lead to interference problems. For example, leakage of a local signal for transmission may disrupt reception of a radio frequency signal in a reception mode. This is particularly problematic when advanced modulation schemes are used that are more susceptible to noise. If local leakage is too high it may be difficult to provide successful Direct Current (DC) compensation during a reception operation, which can lead to degradation of data throughput or loss of a data connection. Different data classes, for example as specified by 3rd Generation Partnership Project (3GPP) standards, may require different signal to noise ratios (SNRs) and/or signal interference sensitivities and so the degradation of data throughput effect of leakage may vary according to said classes.
A number of solutions to the problem of interference in TDD systems have been proposed. In certain Global System for Mobile Communication (GSM) systems guard bands are used between transmit and receive time slots to allow appropriate components to be switched on and off without settling effects. However, the use of guard bands restricts the data rates for communication.
U.S. Pat. No. 5,483,679 describes a radio communication apparatus that uses a common carrier frequency for transmission and reception operations, which are selectively carried out in the apparatus. Three local oscillators are used: a modulation local oscillator, a common local oscillator and a subsidiary reception local oscillator. During a reception operation, a frequency divider coupled to the modulation local oscillator is put into an inactive state to isolate the modulating local oscillator and the modulator.
US 2009/0286501 A1 describes a multi-radio device that may operate in a TDD mode. Pairs of frequency adjustable diplexers are arranged in series on transmission and reception paths and are configured by control signals based on a particular use case.
There is a need for an improved apparatus and method for time division duplex communication that minimises interference and leakage between transmit and receive radio frequency signals.